At the present time, there are a wide variety of braking systems that have been proposed for railroad or other guided vehicles. Most of these known systems operate on an open loop, i.e. there is no feedback of the braking rate which is used to control the braking effort.
An example of a brake control system is found in the Frasier U.S. Pat. No. 4,344,138. This discloses a digital air brake control system. The brake system includes an on-board computer in each car. This computer has stored in it a pressure for the pneumatic braking system, corresponding to a pressure which is 10% below the pressure that would cause the wheels to lock and slide for an empty car. However, the system depends entirely on normal redundancy for safe braking distance. The braking is effectively applied in an open loop. When emergency braking is called for, the computer provides a pressure in the brake cylinder which is 105% of the recorded pressure mentioned previously, i.e. a pressure just below that necessary to cause the wheels of an empty car to lock and slide.
In many conventional railroad braking systems, the service brake is provided with an open loop control, to enable it to be controlled by an operator. Additionally, an emergency brake is provided. The emergency brake is actuated either by the operator, or automatically by signalling, in the event that it is necessary to stop the vehicle in as short a distance as possible.
However, there are some applications for which such a braking system is unsuited. In particular, for Urban Transit systems, such as Intermediate Capacity Transit Systems (ICTS), it is desirable to run the ICTS trains at short headways. Thus, ICTS vehicles produced by the assignee of the present invention employ Linear Induction Motors, which provide a high performance. To make full use of this performance, a high service braking level is used. The service brake provides a constant 1 m/s.sup.2 braking rate.
The braking of such an ICTS vehicle is controlled by a Vehicle On Board Computer (VOBC). The VOBC is programmed with a safe stopping distance, which is dependent on the relative levels of the service braking used and emergency braking effort available. To assure a safe stopping distance, a conservative approach is taken when determining the braking available from the emergency braks. Thus, one assumes a heavily laden train, and poor adhesion between the wheels and the rails. A conventional emergency brake is open looped depending only on spring application of disc brakes and the like, and the application of track brakes.
An unfortunate consequence of these operating conditions is the excessive level of emergency braking occurring. Since the level of service braking is high, although always less than the minimum emergency braking available, and since the VOBC assumes a conservative safe stopping distance, emergency braking is called for relatively frequently. This leads to two unacceptable operating conditions. Firstly, emergency braking will often cause the wheels to lock and slide, resulting in wheel flats. At best, this results in noisy vehicle operation, and can require frequent replacement or repair or wheel sets, which is expensive. Secondly, passenger comfort and safety are compromised; excessive acceleration and jerk rates are not acceptable.